Torrefaction and partial pyrolysis of material for fuel pellet production

ABSTRACT

The present application discloses a process for the preparation of fuel pellets, said process comprising the steps of a) subjecting a feedstock (e.g. a biomass material) to a torrefaction and partial pyrolysis step at a temperature in the range from 250° C. to 500° C., whereby a solid char and volatile fraction are obtained, said volatile fraction comprising a tar fraction; b) at least partially condensing the volatile fraction so as to obtain a tar-rich fraction, wherein the tar-rich fraction is condensed onto the solid char; and pelletization of the combined solid char/tar-rich fraction so as to obtain said fuel pellets. Novel fuel pellets are also disclosed.

FIELD OF THE INVENTION

The present invention relates to the field of fuel pellets production based on various sources of feedstock, e.g. biomass or waste.

BACKGROUND OF THE INVENTION

Bergman and Kiel, “Torrefaction for Biomass Upgrading”, 14^(th) European Biomass Conference & Exhibition, 17-21 Oct. 2005, discloses a process of torrefaction of biomass, separation of the volatiles, and cooling of the torrified biomass. It is suggested that the torrified biomass may undergo size reduction and pelletization.

Bergman et al., “Torrefaction for biomass co-firing in existing coal-fired power stations—“Biocoal””, ECN-C-05-013, Energy research Centre of the Netherlands (ECN), 2005, discloses a process of torrefaction of biomass at a temperature of about 280° C., wherein the torrified biomass is cooled and the torrefaction gas is combusted and used for drying the biomass and as a heat supplement for the torrefaction process.

Bergman, “Combined torrefaction and pelletisation—The TOP process”, ECN-C-05-073, Energy research Centre of the Netherlands (ECN), 2005, discloses a process of torrefaction of biomass at a temperature of 250-300° C. and subsequent pelletization.

In Gilbert et al., “Effect of process parameters on pelletisation of herbaceous crops”, Fuel 88 (2009), 1491-1497, a study of pelletisation under various conditions is reported. It was concluded that torrifaction of grass was not an attractive pre-process as the pellets were very brittle and possessed little mechanical strength and reduced bulk density. It was mentioned that heavy pyrolysis oil has a potential for use as a binding material which can significantly increase the strength and durability of the pellets.

WO 2010/129988 A1 discloses a process for the preparation of fuel pellet, wherein a feedstock is subjected to torrefaction and/or partial pyrolysis at temperature in the range from 250 to 500° C., whereby a solid char and a volatile fraction are obtained. The volatile fraction is used for heating of a mixer vessel. The condensed tar may subsequently be combined with the solid char.

EP 2,287,278 A2 discloses torrefaction of biomass, whereby a solid fraction is directed to a cooler. A rotary valve ensures that the volatile is not allowed to enter the cooler, but is instead fed to a combustion unit.

US 2009/007484 A1 discloses an apparatus and process for converting biomass feed materials into reusable carbonaceous and hydrocarbon products. The biomass may be torrified and the volatile fraction is condensed in one or a series of condensors. The solid material may be pelletized.

SUMMARY OF THE INVENTION

In this text char is defined as biomass or waste with a high organic fraction that has been exposed to a temperature of minimum 200° C.

The present invention provides a process for providing fuel pellets based on biomass or waste that can be optimized for use in power plant boilers (grate, fluid bed or suspension fired), district heating boilers, small pellet stoves, industrial process furnaces, kilns and boilers, small-scale heating devices, and barbeque grills. The pellets may be utilized as a global trading product. Process steps to control pellet heating value density, pellet milling properties, particle size in pellet and pellet ash properties may be included. From the end-user's point of view, the following pellets properties are attractive, namely i) a high heating value density to minimize transport costs, ii) a high pellet stability and hydrophobic properties of the pellets which make handling simple, minimized dust problems, and thereby reduce the risk of self-ignition and provide the option of out-door storage even in wet climates; iii) the option of easy grinding of the pellets in a mill e.g. a coal mill to obtain a small particle size; and iv) acceptable pellet ash properties so that ash deposition, corrosion and flue gas cleanings equipment interference are minimized and residual product utilization is possible.

Hence, the present invention provides a process for the preparation of fuel pellets, said process comprising the steps of

-   -   a) subjecting a feedstock (e.g. biomass material) to a         torrefaction and partial pyrolysis step at a temperature in the         range from 250° C. to 500° C., whereby a solid char and a         volatile fraction are obtained, said volatile fraction         comprising a tar fraction;     -   b) simultaneously or sequentially i) at least partially         condensing the volatile fraction so as to obtain a tar-rich         fraction, wherein the tar-rich fraction is condensed onto the         solid char, whereby the solid char is combined with at least a         part of the tar-rich fraction; and     -   c) pelletization of the combined solid char/tar-rich fraction so         as to obtain said fuel pellets.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates the overall process for the preparation of fuel pellets. The shown process can provide torrified pellets with optimal properties for different uses. The process may include several steps as shown in FIG. 1 and the process can be implemented by use of a variable number of process steps. The total process may include torrefaction, size reduction of torrified material, cooling and condensation of tar and separation of gas, possible addition of additives and pelletization. The needed heating (for process A on FIG. 1) can possibly be provided by combustion of the evolved gasses or by another energy source. The heat transfer to the feedstock can be provided by a hot metal surface, by superheated steam, by bed material e.g. sand, by a flue gas depleted of oxygen or by a material such as ceramic or metal balls or element of irregular shape.

FIG. 2 illustrates one possible set-up for implementing the invention. A screw unit and a pelletizer are combined. The feedstock is transported into the pelletizing unit by the screw feeder. In the first part of the screw feeder, the feedstock is heated to a pre-set temperature to release tar and gas and obtain more fragile properties of the solid char. The residence time is defined by the rotation velocity of the screw feeder and the dimension of the screw unit. Further downstream of the screw, the solid char is cooled and tar is condensed on (i.e. combined with) the solid char, while the remainder of the gas proceeds further. Finally the combination of the tar and solid char is pelletized.

FIG. 3 illustrates another possible set-up for implementing the invention. In this case wet feedstock is dried in a rotation steam dryer that is followed by a rotary kiln steam pyrolysis unit. The evolved products are cooled to 110° C. whereby the tar is condensed on the char and the gas and steam are directed to a condenser unit. The gas leaving the condensation unit is combusted to provide steam for the dryer and the pyrolyser. The cooled char and tar are finally pelletized.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, the present invention provides a process for the preparation of fuel pellets in which a feedstock undergoes torrefaction and partial pyrolysis, and wherein produced tar is condensed onto solid char followed by pelletization.

The Feedstock

The process of the present invention may be applied using a wide variety of feedstock, e.g. a biomass material or waste, including herbaceous biomass such as straw and grains, wood biomass including hard and softwood, as well as in principle all waste types with a significant (>10 wt %) organic fraction, or any mixtures of such feedstock. Preferably, the feedstock has an organic content of at least 15 wt %, such as at least 20 wt %, e.g. at least 40 wt %, or at least 60 wt %.

In one currently preferred embodiment, the feed stock is a biomass material. Many preferred biomass materials have an organic content of at least 80 wt %, such as at least 90 wt %.

Preferred types of feedstock include straw, grains, hard wood, soft wood, and dried sewage sludge. In some embodiments, the feedstock is wood (typically ash content 0.3 to 3 wt %), annual biomass (typically ash content 4 to 10 wt %), or more variable organic waste materials such as waste wood or dried sewage sludge.

Preferably, the water-content of the feedstock is reduced to 2-15 wt % prior to the torrefaction and partial pyrolysis process of step a) below. Reduction of the water content may be obtained in the first process step by steam drying, heating, compression or centrifugation.

Hence, in one embodiment of the process, step a) (see immediately below) is preceded by a drying step wherein the water-content of the feedstock is reduced to less than 10 wt %.

Step a)

After a possible drying the first step of the process includes a combined torrefaction and partial pyrolysis process (see also Process A in FIG. 1).

The torrefaction process is carried out by heating the feedstock in a suitable reactor in an inert atmosphere or an atmosphere with less than 0.5 Vol % O₂ up to a temperature from 200° C. to 300° C. The atmosphere typically consists of the evolved volatiles, N₂, CO₂, steam or a flue gas depleted of oxygen. The residence time of the feedstock in the reactor at temperatures for torrefaction is typically between 0.5 seconds and 2 hours.

Typically, a solid char product yield after torrefaction of 50 to 90 wt % is obtained containing 70-90% of the feedstock heating value. The residual product is a volatile fraction (a gas) rich in CO, CO₂ and water with smaller contents of H₂ and some light hydrocarbons, and possible small amounts of tar.

At higher temperatures, i.e. from 300° C. to 500° C., the process is defined as a partial pyrolysis process. The residence time of the feedstock in the reactor at temperatures for partial pyrolysis is typically from 0.5 second to 1 hour.

The solid char product yield after partial pyrolysis is typically from 15 to 85 wt % depending on process conditions (temperature, heating rate, residence time). The evolved volatiles (i.e. the volatile fraction) contain both a gas and a condensable fraction of tar rich in oxygenated hydrocarbons. The tar yield can be in the range from 2 to 65 wt % of the feedstock depending on operation conditions.

It should be understood that the border-line between torrefaction and partial pyrolysis is somewhat theoretical, because it is found that the volatile fraction of a torrefaction process already from about 250° C. may comprise tar.

The present invention combines the torrefaction and partial pyrolysis so as to obtain a suitable amount of tar. Hence, the optimal torrefaction/pyrolysis reactor operation temperature is a compromise between two objectives. The temperature shall be sufficiently high to obtain a sufficient yield of tars and thereby to obtain pellets with adequate quality. Also the char yield shall be as high as possible to obtain a maximum of the feedstock energy content transferred to the fuel pellets. Generally, the char yield decrease and the tar yield increase with increasing reactor temperature. It is not possible to define a generally applicable optimal reactor temperature for all types of feedstock. However, previously conducted studies indicate that the optimal temperatures may be in the range of 250 to 500° C. The actual optimal reactor temperature is dependent on the applied feedstock and reactor type.

However, in some preferred embodiments, the torrefaction and partial pyrolysis involves that the feedstock is subjected to a maximum temperature in the range from 250° C. to 500° C., such as from 260° C. to 490° C., e.g. from 270° C. to 480° C., or from 280° C. to 475° C., or from 290° C. to 470° C., or from 300° C. to 460° C., preferably from 310° C. to 450° C., or from 320° C. to 450° C., or from 330° C. to 450° C., or from 340° C. to 450° C., or from 350° C. to 450° C. In other embodiments, the feedstock is subjected to a temperature in the range from 250° C. to 400° C., such as from 260° C. to 390° C., e.g. from 270° C. to 380° C., or from 280° C. to 360° C., or from 290° C. to 350° C.

The combined torrefaction and partial pyrolysis is typically allowed to proceed for a total period from 2 seconds to 2 hour, such as from 10 seconds to 90 minutes, such as from 4 minutes to 90 minutes, or from 6 minutes to 70 minutes, e.g. from 8 minutes to 50 minutes.

A possible method to control the quality of the obtained pellets could be to use an instrument that determines the amount of condensable products in the volatiles fraction. The instrument could determine the amount of condensed material by cooling the volatile fraction to e.g. 110° C.

The torrefaction process and the partial pyrolysis process may be run as separate processes in the same or separate reactors, however, preferably, the processes are run sequentially, e.g. by using a temperature gradient. The processes may be implemented with co-current flow conditions (as illustrated in FIG. 1).

Hence, in some embodiments, the feedstock is heated for up to 2 hours. Within this embodiment, the exit temperature at completion of the torrefaction and partial pyrolysis process typically is in the range from 300° C. to 450° C.

A heat source is needed to facilitate the torrefaction and partial pyrolysis process. Heat may be supplied by heat transfer through a metal wall, by an intermediate heat carrier such as sand, ceramic, concrete or metal balls, steam, CO₂ or by a flue gas nearly depleted of oxygen. Heat can be generated by using the gas developed in process step a), by using heat from other processes or by using a separate fuel supply.

A possible size reduction of the char may be performed (see Process B in FIG. 1) in order to obtain a more homogeneous char fraction with reduced particle size. This could be as a separate process step or integrated with the torrefaction and/or partial pyrolysis processes.

The output stream from step a) (see Process A (and Process B) in FIG. 1) is a solid char and the volatile fraction (volatile constituents at the exit temperature). The volatile fraction comprises gasses, water and tar. In the present context, “gasses” are defined as the fraction of the volatiles which is still in the gas phase at 25° C. and 1 atm.

One interesting fraction of the volatile fraction is the tar fraction, which will be discussed further in connection with step b) below.

The torrefaction/pyrolysis process (possibly also including the subsequent condensation step, see step b)) can be implemented by use of a range of different reactors, some examples are provided:

-   Single or multiple screw reactors. An example is shown in FIG. 2.     The process heat may be provided by external heating of the screw     channel wall, by heating the screw or by injection of superheated     steam. -   Ball mills or rotary kiln type reactors. The feedstock can be     simultaneously grinded and heated. Heat for the process can be     provided with external heating, steam, heating of metal or ceramic     balls, by other heat carrying materials or by injection of a     sub-stoichiometric hot flue gas. An example of a possible plant for     production of torrified pellets is shown in FIG. 3. Both the     feedstock drying and torrefaction/pyrolysis units are based on     rotary kiln technology. -   Fluidized bed reactors, bubbling bed or circulation fluidized     reactors. Heat can be provided by combustion in a separate secondary     bed and hot solids are then mixed with the feedstock in a primary     bed. -   Fixed or moving bed reactors. The fuel is exposed to a counter flow     of hot flue gas. The exit gas is cooled whereby tar is provided.     Char is removed from the bottom part of the reactor. Char and tar     are mixed before pelletization. Hot flue gas is provided by     combustion of a part of the evolved gas and char.

Step b)

An essential feature of the step b) (see Process C in FIG. 1) performed on the volatile fraction and the char from step a) is to partially condense the volatile fraction so as to obtain a mixture of tar and the solid char. This can be realized as a cooling of the complete product stream (i.e. the solid char and volatile fraction). In this way the tar will condense in the section of the reactor wherein the solid char is also present whereby the tar effectively will condense onto the solid char.

The tar-rich fraction is typically condensed when cooling from the exit temperature of step a) (i.e. the torrefaction/partial pyrolysis temperature (such as about 350° C.)) to a temperature of 20-150° C., such as 50-150° C.

Water can be condensed upon cooling to a temperature below 100° C. i.e. below the water drew point temperature. In some embodiments, it is desirable to allow water to become condensed together with the tar-rich fraction in that the presence of water will facilitate the pellet formation (step c)).

Hence, in one embodiment, the cooling is conducted by cooling the volatile fraction from the exit temperature of step a) to at temperature of around 100° C. (e.g. in the range from 50° C. to 150° C.).

Another feature of step b) is to combine the tar-rich fraction (possibly including water) with the solid char. It has been found, that the combination of solid char and the tar-rich fraction will provide benefits with respect to the pelletization and with respect to the properties of the final fuel pellets.

By controlling the cooling temperature of the complete product stream from step a) (i.e. the solid char and the volatile fraction), a controlled mixture of the solid char and the tar-rich fraction (possibly including water) can be obtained. Hence, by cooling of all products in one step tar and water can be efficiently mixed with the char.

Thus, in one preferred embodiment, solid char and the volatile fraction are simultaneously cooled, whereby the solid char is combined with at least a part of the tar-rich fraction. This is e.g. illustrated in FIG. 2.

Hence, in another embodiment, the cooling is conducted by cooling the solid char and the volatile fraction from the exit temperature of step a) to a temperature of around 100° C. (e.g. in the range of 50° C. to 150° C.) while allowing the condensed tar-rich fraction to become mixed with the solid char.

In some embodiments, any gasses from the volatile fraction from which the tar-rich fraction is condensed, may be combusted so as to provide energy to any drying of the feedstock or to the torrefaction and partial pyrolysis process. Hence, the evolved gas may be used to provide heat for, e.g., process step a).

The cooling step could depending on temperature be utilized for power or heat production, e.g. by heat exchange with appropriate water or steam cycles.

For some types of feedstock (typically alkali rich feedstock) and for some applications of the fuel pellets, it may be advantageous to combine additives (see Process D in FIG. 1) with the solid char (and the tar), which in a combustion process can bind alkali metals or other species and make them less harmful.

Hence, in some embodiments, it is—for the purpose of making optimal pellets for different combustion and gasification units—advantageous that the pellets are formulated by addition of additives (see Process D in FIG. 1) prior to pelletization. The additives may be clay minerals, lime stone, bleaching soil, sewage sludge or other waste products. Generally materials containing more than 5 wt % of one or several of the following elements may be used: S, P, Al, Si and Ca. The additives are provided so as to modify the properties of the pellets, e.g. such that ash deposition and corrosion problems during pellet combustion are minimized. Additives promoting/catalyzing the tar curing process may also be added.

Examples of pellets formulations may include:

-   A. Prevention of slagging in the bottom part of small scale pellet     stoves. Often melting of bottom ash appears in the bottom part of     pellet stoves whereby fuel feeding is disturbed. An addition of     calcium containing species may increase the melting temperature of     the produced bottom ash. Addition of limestone to obtain a molar     ratio of Ca/K more than 2 in the fuel pellets will often be     sufficiently to prevent bottom ash slagging. -   B. When biomass based pellets are used in large dust fired power     plant boilers problems with severe deposit formation on the super     heaters may be observed. This makes problems both with accumulation     of deposits and corrosion of super-heater tubes. Addition of     sufficiently amounts of minerals rich in Si and Al may remedy those     problems. Obtaining a fuel pellet with a molar ratio of more than     2.5 of (Si+Al)/(K+Na) may significantly reduce problems.

Any additives may be combined with the solid char before, in combination with, or after combination of the solid char with the tar-rich fraction. In some embodiments, the additives may even be fed together with the feedstock.

Step c)

In step c) of the process (see Process E in FIG. 1), the combination of the solid char (preferably in particulate form after grinding) which is combined with the tar-rich fraction (possibly including water) and any additives (see step b) is pelletised.

Keeping of the material at a temperature in the range from 50° C. to 100° C. of the pelletizing process may increase pellet stability and hardness.

The pelletizing is conducted using conventional equipment, e.g. an Andritz sprount pellet mill, using conventional conditions.

The pelletizing may be followed by a curing step in order to harden the pellets, e.g. by curing the tar.

Preferably step a) and step b) of the process are run as a continuous process. In some interesting embodiments hereof, step a), step b) and step c) of the process are run as a continuous process.

Particular Embodiments

In one embodiment, the invention relates to a process for the preparation of fuel pellets, said process comprising the steps of

-   a) subjecting a biomass material selected from wood to a     torrefaction and partial pyrolysis step at a temperature in the     range of 250-400° C., e.g. 300-350° C., by heating the biomass     material in a reactor whereby a solid char and a volatile fraction     are obtained, said volatile fraction comprising a tar fraction; -   b) cooling the solid char and the volatile fraction from the exit     temperature of step a) to a temperature in the range of 20° C. to     150° C., e.g. 50-150° C., while allowing the condensed tar-rich     fraction and any water to be combined with the solid char; and -   c) pelletization of the combined solid char/tar-rich fraction so as     to obtain said fuel pellets.

The product can advantageously be stored and transported with high stability and the pellets can be used as fuel in a pulverized fired power plant boiler.

Pellets

The primary demands for an adequate pellet quality is a pellet that is hydrophobic and does not fragment significantly during transportation. Hence, the pellets should also have suitable mechanical strength, e.g. defined as the tensile strength thereof. The tensile strength can be measured using a tensometer for compression of a pellet in the radial direction, cf. the method described by da Rocha SSHF, “Mechanical evaluation for the quality control of biomass pellets and briquettes. In: Proceedings of the second world conference on pellets, Jönköping, Sweden; 2006, 183-187.

It appears that useful pellets preferably have a tensile strength of at least 100 kPa, such as at least 200 kPa, e.g. at least 300 kPa. Very attractive pellets are those having a tensile strength of at least 400 kPa, such as at least 500 kPa, or at least 600 kPa, or at least 700 kPa.

Hence, it is believed that the pellets obtained by the above process are novel as such. Hence, the present invention also provides a fuel pellet comprising a solid char, tar, and, optionally, one or more additives, said solid char and said tar being obtained by torrefaction and partial pyrolysis of a feedstock at a temperature from 250° C. to 500° C. Preferably, the pellet has a tensile strength of at least 100 kPa.

The pellets prepared according to the invention can be grinded with low energy consumption and is thereby optimal to use in suspension fired boilers. Moreover, the pellets can be stored under out-door conditions on moist regions of the world, e.g. in the Scandinavian countries.

Use of Pellets

The pellets prepared according to the invention can be provided to a national or an international market with end-uses in: power plant boilers (grate, fluid or suspension fired), district heating boilers, small pellet stoves, industrial process furnaces, kilns and boilers, small-scale heating devices, and barbeque grills. 

1. A process for the preparation of fuel pellets, said process comprising the steps of a. subjecting a feedstock to a torrefaction and partial pyrolysis step at a temperature in the range from 250° C. to 500° C., whereby a solid char and volatile fraction are obtained, said volatile fraction comprising a tar fraction; b. at least partially condensing the volatile fraction so as to obtain a tar-rich fraction, wherein the tar-rich fraction is condensed onto the solid char, whereby the solid char is combined with at least a part of the tar-rich fraction; and c. pelletization of the combined solid char/tar-rich fraction so as to obtain said fuel pellets.
 2. The process according to claim 1, wherein the condensation of the volatile fraction in step b) is conducted at a temperature of in the range from 50° C. to 150° C.
 3. The process according to claim 1, wherein the process further comprises size reduction of the solid char, wherein the size reduction takes place as a part of step a) and/or immediately subsequent to step a). 4-6. (canceled)
 7. The process according to claim 2, wherein the process further comprises size reduction of the solid char, wherein the size reduction takes place as a part of step a) and/or immediately subsequent to step a).
 8. The process according to claim 1, wherein the water in an amount of up to 0-15 wt % by weight of the solid char is combined with the solid char in step c).
 9. The process according to claim 2, wherein the water in an amount of up to 0-15 wt % by weight of the solid char is combined with the solid char in step c).
 10. The process according to claim 3, wherein the water in an amount of up to 0-15 wt % by weight of the solid char is combined with the solid char in step c).
 11. The process according to claim 7, wherein the water in the amount of up to 0-15 wt % by weight of the solid char is combined with the solid char in step c).
 12. The process according to claim 1, wherein additives are provided in the pellets so as to modify the properties of the pellets.
 13. The process according to claim 2, wherein additives are provided in the pellets so as to modify the properties of the pellets.
 14. The process according to claim 3, wherein additives are provided in the pellets so as to modify the properties of the pellets.
 15. The process according to claim 8, wherein additives are provided in the pellets so as to modify the properties of the pellets.
 16. A fuel pellet comprising a solid char, a tar, and, optionally, one or more additives, said solid char and tar being obtained by torrefaction and partial pyrolysis of a feedstock at a temperature from 250° C. to 500° C., wherein said pellet has a tensile strength of at least 100 kPa. 